Tamiya x Clear Coat

If I want to clear coat a Tamiya X paint will I need to flatten it first for a glossy finish?

No, you can apply X22 over flat or gloss finishes.

Will it still work if I use halfords laquer?

If you mean the Halfords lacquer in an aerosol, I think it's a little too strong over Tamiya acrylics and may lift the finish.